In order to accurately control a rotation operation of a rotary electric machine, there are required rotor position information of the rotary electric machine and current information about a current flowing in the rotary electric machine. Conventionally, the rotor position information is obtained by separately mounting a rotor position sensor to the rotary electric machine. However, it is considerably disadvantageous to separately mount the rotor position sensor from the viewpoints of cost reduction, space-saving, and improvement in reliability, and thus there is a demand for a sensorless method which does not require the rotor position sensor.
As a sensorless control method not requiring a rotor position sensor in a rotary electric machine, there are mainly the following methods: a method in which a rotor position of a rotary electric machine is estimated based on an induced voltage of the rotary electric machine; and a method in which a rotor position of a rotary electric machine is estimated utilizing a saliency. Since the magnitude of the induced voltage used in the former method is characteristically proportional to a speed of the rotary electric machine, the induced voltage is decreased at a zero-speed or in a low-speed range, and an S/N ratio deteriorates. Therefore, it is difficult to estimate the rotor position of the rotary electric machine. On the other hand, in the latter method which utilizes the saliency, a rotor position estimation signal for estimating a rotor position of a rotary electric machine needs to be injected to the rotary electric machine, however, it is advantageous that the rotor position of the rotary electric machine can be estimated regardless of the speed of the rotary electric machine. Therefore, the sensorless control method utilizing the saliency is employed for position detection in the zero- or low-speed range.
Conventionally, in the sensorless control method utilizing the saliency, in order to generate high-frequency signals (for detecting a position of a magnetic pole rotor), which are different from a driving frequency for a rotary electric machine, as signals for estimating a rotor position, carrier signals of three phases, which each have an arbitrary frequency and are generated by a carrier signal generator, are subjected to phase shift by a phase shifter, and phases of a V-phase and a W-phase are shifted by an angle Δθ and an angle 2Δθ, respectively, with respect to a U-phase. The phase-shifted signals are compared to voltage command values by using a comparator to generate switching signals which are then inputted to an inverter circuit. In the inverter circuit, high-frequency currents of three phases, which are generated due to driving of the rotary electric machine by the switching signals, are extracted through a band-pass filter (BPF). Next, the high-frequency currents of three phases are converted by a coordinate converter to an α-axis, a β-axis, an α′-axis, and an β′-axis, and peak values of the current components are extracted and then subjected to an averaging process by using an absolute value calculation unit and a low pass filter, and then θ is estimated by a magnetic pole position calculation unit (see Patent Document 1, for example).
Further, as another conventional method which utilizes the saliency, there is known a method in which a d-q axis rectangular coordinate system, which rotates synchronously with a rotor or a magnetic flux vector, is used as control coordinates, and a high-frequency signal is superimposed on a d-axis excitation current command value, so as to detect currents flowing to the rotary electric machine. Two-phase currents are obtained by three-phase/two-phase conversion of the above currents. The square of the amplitude of a vector of each detected current, which is the square sum of the currents of two phases, is calculated, and the sum between the square of the d-axis excitation current command value and the square of a q-axis torque current command value, i.e., the square of the amplitude of a command current vector, is calculated, and the square of the amplitude of the command current vector is subtracted from the square of the amplitude of the detecting current vector. Based on a value obtained by the subtraction, an error from the control coordinates is calculated, whereby a rotor position in the rotary electric machine is estimated (see Patent Document 2, for example).
On the other hand, current information about a current flowing to a rotary electric machine is conventionally detected by arranging a plurality of current sensors between voltage application means such as an inverter or the like and the rotary electric machine, and by detecting, by using the current sensors, the rotary electric machine current flowing between the voltage application means and the rotary electric machine. For example, in the case of a three-phase AC rotary electric machine, rotary electric machine currents of at least two phases, among three phases, are detected using two current sensors. However, arrangement of a plurality of current sensors requires extra costs. Thus, for the sake of reduction in costs of the current sensors, there is a method in which only one current sensor is used to detect a value of a DC bus current flowing between a DC voltage source, which is an input source to voltage application means such as an inverter or the like, and the inverter, so as to calculate and identify the phase, of the rotary electric machine, in which the current is flowing at the time of the detection in accordance with the difference in switching patterns of switches of respective phases of the voltage application means such as the inverter or the like.
The above method enhances the reduction in costs of the current sensors. However, when voltage command values are overlapped with or close to each other, for example, when the modulation percentage of fundamental waves for driving a rotary electric machine is small, or when command values of two phases, among voltage command values of three phases, are overlapped with each other, then switching elements of the respective phases of voltage application means, such as an inverter or the like, perform switching substantially simultaneously, which leads to substantially no difference in the switching pattern. Therefore, the method has a problem in that it is difficult to identify the phase, of the rotary electric machine, in which a current is flowing.
To solve the above problem, conventionally proposed is a method which is combined with the sensorless control, and in the method, three phase carriers are prepared to estimate a rotor position by using high-frequency currents generated by three phase carrier modulation, and the three phase carriers utilized for estimation of the rotor position are utilized to generate difference in switching patterns of switching elements of the respective phases of voltage application means, such as an inverter or the like, thereby detecting a DC bus current even when the modulation percentage of fundamental waves for driving the rotary electric machine is small. Accordingly, it is possible to identify the phase, of the rotary electric machine, in which a current is flowing, and to calculate the current flowing to the rotary electric machine (see Non-patent Document 1, for example).    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-52193    [Patent Document 2] Japanese Patent No. 3707528    [Non-patent Document 1] Initial Rotor Position Estimation Characteristics of Mechanical-Current Sensorless IPM Motor Using PWM Harmonics Detected in DC-Bus Current (I.E.E. Japan Industry Application Society Conference 1-100 (2005))